Several GNSSs exist or are created in the world. Those are live systems: Global Positioning System (GPS, USA), Global Navigating Satellite System (GLONASS, Russian Federation), Galileo (European Space Agency, ESA) which is under creation, and BeiDou or Compass (Peoples Republic of China). The present invention can be used in navigation receivers of all of these or other GNSSs.
Using navigation receivers in urban canyon conditions results in frequent missing a line-of-sight of satellites. Power of received signals thus becomes low or vanishes. In these conditions, traditional autonomous signal tracking in a close-loop system by means of delay lock loop (DLL) and a frequency lock loop (FLL) is often interrupted.
Tracking satellite signals in an open-loop system (“Open-Loop Tracking”, [U.S. Pat. No. 6,633,255]) is realized by means of calculating, for each satellite, predictions of delay and Doppler frequency measurements based on navigation solution, forming two-dimensional (delay and Doppler frequency) power grid with the center defined by the forecast of measurements, obtaining corrections for the prediction, and forming measurements as the sums of corrections and the prediction. Such tracking ensures operation for low signal/noise ratios.
The signal loss from a part of satellites in the system realizing tracking in an open loop is not critical while navigation solution exists, i.e., measurements remain based on signals from at least 3-4 satellites.
Presence of signals coming by direct and reflected paths results in occurrence both of delay and Doppler frequency errors. FIG. 1 illustrates the exemplary diagram of multipath signal propagation, results in distortions of measurements:                for a static receiver or receiver moving in parallel with reflecting surface (FIG. 1a), delay measurement errors occur only, because rates of change of distance to a satellite for direct and reflected paths are equal;        for a receiver moving in the direction of a reflecting surface (FIG. 1b), delay measurement errors are added to Doppler frequency measurement errors, because in this case rate of change of distance to the satellite by a reflected path differs from rate of change of distance to the satellite by a direct path.        
Methods are known which provide reduction of pseudorange measurement errors resulting from presence of reflected paths. They are generally: a narrow correlator [Theory and Performance of Narrow Correlator Spacing in a GPS receiver. A. J. van Dierendonck, Pat Fenton, Tom Ford. Journal of The Institute of Navigation, Vol. 39, No. 3, 1992.], strobe methods for replica forming for a pseudo-random sequence (PRS) of a signal by way of calculation of autocorrelation function (ACF) [U.S. Pat. No. 7,436,356, filed 24 Mar. 2006], methods based on the usage of ACF leading edge, calculation of pieces crossing on leading and trailing edges and ACF samples weighting [U.S. Pat. No. 7,436,356, filed 24 Mar. 2006]. These methods are easily realized in inexpensive mass produced receivers and reduce a pseudorange error, but their accuracy is not high.
Also known are multipath mitigation methods based on optimum processing of the multipath signal including measurement of signal parameters for direct and reflected paths with applying a method of maximum likelihood. Suppression of the reflected signals effect is realized by optimum estimating and exception of reflected signals from observations. These methods are used in high cost geodetic receivers and demand considerable expensive hardware for its realization.
Usage of adaptive antenna array for suppression of signals coming by reflected paths requires several receive antennas and additional processing of signals received by them, which also complicates consumer equipment considerably.
The closest to the claimed solution is the method described in the U.S. Pat. No. 6,031,881, published Feb. 2, 2000. Disadvantages of this method consist in necessity of saving input signal samples, quantized in a high frequency (in the patent the sampling rate is indicated as equal 20 MHz for GPS system), as well as in processing these samples in real-time. For realization of such method a great volume of memory and a high rate of processing information are required.
Other disadvantage is limited accumulation time. The reason of limiting accumulation time is modulation of a received signal by unknown data bits of the navigation message in a transmitted signal (every 20 ms for GPS and GLONASS), which leads to limiting the received ratio of carrier signal power C to noise power N0 in the band of 1 Hz C/N0>30 dB Hz.